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Technology I n s t i t u t e Univ. of Connecticut A u g u s t 5, 1965

. , Missile and Space

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z3 '27 (CATEGORY)

NATURE OF THE SPACE EMlIRONMENT

By John C. Ward National Aeronautics and Space Administration

Cleveland, Ohio

I n the broad sense, the word "space" i s a l l inclusive. It includes the Sun, the Earth, and the other planets of the solar system. one hundred b i l l i o n stars or more i n our own galaxie, which we l a b e l the "Milky Way." as nebulae. Certainly there a re b i l l i ons of stars i n this disk-shaped conglomeration, some of which may have planets and l iv ing in t e l l i gen t beings. ment near these b i l l i ons of stars is r ight f o r l i v ing forms, then sure ly l i f e w i l l exist on some of them. This is a challenge f o r the future . Our so l a r system (slide 2) is probably not unique even i n our own galaxy, and we know that there are b i l l i o n s of galaxies i n the universe.

It includes the

It includes a l l the other galaxies of the universe which we see The great nebula i n Andromeda i s shown i n the first slide.

If the environ-

The space environment includes a l l matter or the r e l a t ive lack of matter i n the high vacuum regions between the planetary and s t e l l a r mass concentrations. It likewise includes a l l t rans ien t e x c u r s i p s of matter through these regions, and the influence that these meteoroidal excursions might have on a space sh ip contained therein. X-rays, and electromagnetic radiation. gravity, e l e c t r o s t a t i c a t t rac t ions , and magnetic f i e lds . environment must recognize the probabi l i t ies of cosmic rays, so l a r winds, and lethal radiat ions associated with s o l a r flares.

It includes a l l radiat ion such as l igh t , radiowaves, It includes all force f i e l d s such as

And the space

I shall concentrate on so lar space shown p i c t o r i a l l y i n sl ide 3. you see w h a t i s t o us the most important star of our universe i n the lower right-hand corner. The Sun is, of course, the pr incipal source of energy f o r the so l a r system. It has a diameter of some 860,000 miles. While the in t e rna l temperature of this gigant ic thermonuclear reactor i s about 40,000,000 degrees, the surface temperature is nowhere near this high. of the photosphere is about 6000O K.

Here

The ef fec t ive temperature

The Sun ( s l i d e 4) , however, i s far from being a quiet well-behaved source

There are magnetic storms i n the of heat and l i gh t . Intense storms project g ian t tongues of mater ia l i n t o space a t mill ions of degrees (coronosphere). form of sunspots that vary according t o t he sunspot cycles of about 11 years. I n addition, the Sun ro ta tes on i t s axis carrying these sunspots across i t s surface i n a 27-day period. Each of these periods influence the environment, weather, and atmospheres of the planets. project streams of charged pa r t i c l e s and magnetism outward t o bathe the planets i n a t r ans i en t ly varyfng so lar atmosphere. Thus, each of the planets is bathed

I n addition, the so l a r prominences

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i n the solar atmosphere, which would monotonically decrease in in tens i ty as we proceed outward from the Sun's surface past the planets, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.

The in tens i ty of radiatiorr such as l i g h t and heat vary inversely as the square of the distance from the Sun. A t the Earth 's distance of 93,000,000 miles from the Sun (1 astronomical un i t ) , the so la r energy i s 1.34 kilowatts per square meter. I f t h i s value were t o change by only 10 percent, t he Ear th ' s weather would be dras t ica l ly a l tered, with the poss ib i l i t i e s of melting the Polar i c e caps, and so for th . would do th i s .

A 5-percent change i n the o rb i t a l diameter

The known planets i n the so la r system have distances from the Sun ranging from 0.387 f o r the planet Mercury t o 39.52 A fo r the planet Pluto. energy in tens i ty received by a spacecraft moving across the realms of solar space would therefore vary by a fac tor of 10,000 i n making a t r i p from Mercury t o Pluto. 300 l i g h t minutes) t h a t the probabi l i ty f o r manned f l i g h t beyond the so la r system i s questionable indeed. The nearest star i s about 4.3 l i g h t years away.

The radiant

The planet Pluto i s s o far removed even from Earth (more than

I mentioned e a r l i e r that the space environment included a l l of the force f i e lds contained therein. One of these force f i e lds , the grav i ta t iona l a t t rac t ion , holds very special significance. masses varies as the inverse square of the distance between them:

The force of gravi ty between two

F = L -1 2 r

Because of t h i s force, the planets of the so l a r system are held i n orb i t s about the Sun. or hyperbolic, depending on the r e l a t ive ve loc i t ies and t h e masses of the two bodies. Objects having parabolic o r hyperbolic paths about the Sun, of course, leave the so la r system a f t e r the f i rs t pass-by. The radius vector of a par t icu lar planet sweeps out equal areas i n equal times. of the various planets is proportional t o the cube of t he distance of that planet from the Sun. the centrifugal force associated with the speed of the planet and the curvature of the path just balances the grav i ta t iona l a t t r ac t ion . If the planet were not moving, it would quickly f a l l i n t o the parent body t o become par t of it.

It may eas i ly be shown that these o rb i t s can be e l l i p t i c a l , parabolic,

We are most generally concerned w i t h e l l i p t i c a l paths f o r the planets.

The square of t he o r b i t a l period

Crudely speaking, the planets remain i n o rb i t because

The gravi ta t ional a t t r ac t ion of the Sun and planets does something e l se f o r us. material from space; thus, it builds up l i fe -sus ta in ing atmospheres on the planets and leaves very high vacuum conditions i n t h e regions between the PhIe tS . 6 0 t h a t even the high vacuum regions may be regarded as t h e outer f r inges Of

It serves as a gigantic pump t o remove most of t he non-orbiting

We m u s t recognize, however, t h a t t he grav i ta t iona l pump i s not Perfect,

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A t the surface of the Earth, our atmosphere sustains a pressure of 14.7 pounds per square inch, and the gas has an average molecular weight of about 29.4. assume that the force of gravity, the temperature, and the molecular weight remain constant. top t o bottom equal t o the weight.

We can estimte haw the pressure var ies with a l t i t ude i f we

Take a small volume of gas with pressure d i f f e r e n t i a l fran

But from the gas l a w

or

A d p z - p A g d h

PV = ITRT

(3) p = % T R

Hence, i f T and W are constant (which they are not i n the atmosphere),

* = , w g d h RT P

or

(4 )

(5)

I n the atmosphere, experimentally, the pressure decreases approximately by a f ac to r of 2 f o r each 16,000 fee t . i n t he following table.

The uncertainty of t h i s ru le i s indicated

Pressure, Alti tude, a t m . f t

18,000 34,000 48,000 63,000

112

118 114

1/16

The pressure is roughly 0.01 atmosphere a t 100,000 f ee t . millimeters of mercury at 300 miles.

It i s about lom8

4 , ,

L e t us refine equations ( 2 ) through (5) somewhat. If we think-of the individual constituents of the atmosphere, using p a r t i a l pressures f o r t he oxygen, nitrogen, carbon dioxide, water vapor, e tc . , and define a p a r t i a l density as the mass of t h a t constituent per uni t volume, then without mixing equation (5) would hold f o r t he individual components such as oxygen, nitrogen, e t c . pressures of the individual components :

The t o t a l pressure a t any a l t i t ude would be the sum of the p a r t i a l

J -- - I - ( 6 ) RT RT RT

3? = Pole + P02e + P03e +

The volume percent of any par t icu lar gas j w i l l be

Note, however, that the exponentially decreasing partial pressures of t he components decay a t different rates due t o the d i f fe ren t molecular weights. The heavy gases decay faster than the l i g h t ones. Hence, t he percentage of l i g h t gases at high a l t i t ude w i l l increase. This, of course, i s a complicated way of saying tha t the l i g h t gases of the atmosphere w i l l f l o a t , and the heavy ones w i l l sink.

You w i l l r e c a l l t h a t the equations were derived w i t h the assumption of no Actually the turbulence of the weather band around t h e Earth produces mixing.

so much mixing t h a t the atmospheric composition does not change much up t o an a l t i t ude of about 100 m i l e s . a l t i t ude of perhaps 600 miles blending i n t o a layer of hydrogen a t a considerably higher a l t i tude, ( s l i d e 5) say about 1500 miles.

There is, however, a band of helium a t an

Other phenomena do occur, however, a t t h e lower a l t i t udes . Ozone may be formed. e tc . l a t t e r effects implies t h a t the absorption of t he radiated energy i n the so la r spectrum w i l l a l so vary with a l t i t ude . absorbed, chemical compounds are formed t h a t in tens i fy or modify the absorption character is t ics .

Atomic oxygen can occur below 600 miles; ionizat ion i s probable, The variation i n atmospheric composition with a l t i t u d e including these

Likewise, when t h a t energy i s

You see, the Earth and i t s atmosphere along with the oceans, lakes, and r ivers consti tute a gigantic heat engine i n a so la r rad ia t ion environment. This heat engine has various forms of heat addition, heat t ransfer , mass t ransfer , and other complicating phenomena t o y ie ld the s t a t i s t i c a l l y f luctuat ing conditions leading t o our weather, t o our radio transmission, and t o other

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* phenomena such as northern l i gh t s , magnetic storms, e t c .

Visible radiat ion and a l so radio waves penetrate the atmosphere t o add heat t o the Earth's surface.

levels , but, of course, the density is low.

A l l others are generally absorbed before ' they reach the surface. - The upper atmosphere can reach very high temperature

Infared radiat ion may be transmitted when the skies are c lear bnt would be absorbed by cloud cover. heat of the Earth 's vegetation rad ia tes enough heat t o space on c lear nights t o lower the planet surface temperature below the freezing point, even though the air temperature is above freezing.

Frost, f o r example, results when the inf ra red

The atmosphere i s opaque t o u l t rav io le t rays below 2900 angstrom units. These rays cause the ionizat ion i n the ionosphere and generate the ozone layer at an a l t i t ude of about 15 miles. The u l t r av io l e t rays do not penetrate the ozone layer. i n temperature of the chemosphere, the a i r layer j u s t above the stratosphere. A t higher a l t i tudes , ionization X-ray: absorption, cosmic-ray absorption, and other complicated processes probably i d l u e n c e the heat and mass transfer and radio-wave r e f l ec t ion character is t ics of our atmosphere.

The absorption of the u l t rav io le t rays explains the r i s e

What about radio reception? Radio transmission depends on several d i f fe ren t kinds of waves ( f ig . 6) . rad ia t ion that is d i r ec t ly affected by the presence of the Earth and i ts surface features . space wave. by the troposphere. This re f rac t ion i s due t o changes i n the index of re f rac t ion between the boundaries of air masses of differ ing temperature and moisture content. rad ia t ion that is directed through or ref lected and re f rac ted by the ionosphere. For nearby communication, the ground waves serve. re f rac t ion of the sky waves through the ionosphere i s required. of bending required is greater t o receive the s igna l a t R 1 than a t R2. A sk ip zone of no received s igna l occurs between the l i m i t on the ground-wave propagation distance and the bending l i m i t posit ion of the sky-wave receiver.

The ground wave is that pa r t of the t o t a l

The ground wave has two components, the Earth guided wave and the The tropospheric waves are those that are re f rac ted and r e f l ec t ed

The ionospheric wave i s that part of the t o t a l electro-magnetic

For la rger distances, The amount

The re f r ac t ive index i s close t o unity i n the troposphere so that re f rac t ion is generally s l i g h t . formed by the act ion of u l t r av io l e t l i gh t on oxygen, nitrogen oxide, e t c . Free e lc t rons are therefore available t o change the d i e l e c t r i c constant and t h e conductivity of t he region f o r the re la t ive ly low frequencies of radio transmission. than i n t h e troposphere.

With an increase of a l t i t ude , layers of ionizat ion are

Much grea te r re f rac t ion of radio waves occurs i n the ionosphere

Layers of ionizat ion have been observed by r e f l ec t ion of radio waves. The D ionization layers , a l t i t udes These are the D, E, F1, and F2 layers .

of 40 t o 50 miles, only pe r s i s t i n the d a y t i m e , and the in t ens i ty of ionization is proportional t o the height of the Sun. i s so high that recombination quickly occurs i n the absence of the Sun's

The density of ions and electrons

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radiation. 60 t o 80 miles. The in tens i ty of ionization i s greatest a t loca l noon and i s almost zero a t night. a l t i t ude of about 175 miles. Here the density of ions i s s o low tha t recombination with electrons takes place only slowly. just before sunrise.

The E layer i s largely due t o ionized oxygen atoms a t a l t i tudes of

The F layers have t h e i r maximum ionization a t an

The minimum ionization in tens i ty occurs I n the daytime, there are two F layers.

The degree of ionization depends on the in tens i ty of the so la r radiations. It varies from nighttime t o d a y t i m e , from summer t o winter, with the 28-day period of the Sun's rotation, and with the 11-year sunspot cycle. i s greatest during a sunspot's maximum. influenced by magnetic storms tha t may last several days. These so-called storms include unusual disturbances i n the Earth 's magnetic f i e l d along with accompanying disturbances i n the ionosphere.

The ionization The ionosphere i s a l so strongly

For a given density of ionization, the degree of re f rac t ion becomes l e s s as the wavelength becomes shorter. Eknding i s therefore l e s s at high frequency than a t low frequency. channels, the bending i s so s l igh t t ha t the wave does not re turn t o Earth. This leads t o t he so-called " l ine of sight" l imitat ion.

A t the very high frequencies corresponding t o TV

You w i l l recognize that the conditions i n the ionosphere a re subject t o the whims of the solar weather. By monitoring the conditions of t he ionosphere with radio wave ref lect ions, the long-distance communication networks can choose frequencies for best reception by use of extensive correlat ions. The Bureau of Standards a l so publishes prediction charts 3 months i n advance on the usable frequencies above 3.5 megacycles f o r long-distance communications.

The use of s a t e l l i t e s and high frequencies allows our long-distance radio communication systems t o be f r ee from the vagaries of the Sun. Because the wave would be transmitted through the atmosphere, the system would be dependable i r respect ive of s o l a r distmbances,seasons, t i m e of year, sunspot cycles, e t c . Telstar, Relay, and Early bind a re dramatic experiments demonstrating the f e a s i b i l i t y of such communication systems.

There are numerous poss ib i l i t i e s f o r satel l i tes t o serve as a l i n k i n a communication system. The first used i s the message r e l ay s a t e l l i t e as de- picted on s l ide 7. The s a t e l l i t e contains a tape recorder. Messages t rans- mitted from A when the s a t e l l i t e i s i n view a re relayed t o the ground at point B. This system was used t o deliver Eisenhower's C h r i s t m a s greeting t o the world from a Discoverer s a t e l l i t e . could feasibly a l s o be a l i nk t o a rapid m a i l system. par ts of the globe would be covered about four t i m e s per day.

It has appl icat ion f o r mi l i t a ry dispatches and Using polar orb i t s , a l l

Passive s a t e l l i t e s can serve i n a communication network i f there are enough of them. I n t h i s system, the s igna l t ransmit ted from a ground s t a t i o n t o a s a t e l l i t e i s ref lected down t o other ground s ta t ions . type include the Echo balloon configurations, t he wire dipoles of Project Westford, and contemplated shaped balloons t o improve the s igna l re f lec t ion

S a t e l l i t e s of t h i s

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strength. .

The passive s a t e l l i t e s m u s t generally be at f a i r l y low a l t i t udes i n order t o give suf f ic ien t s igna l s t rength at the receiver. the strength of the s igna l transmitted t o t he satellite, even with a very good

This problem a r i se s because

I * anke rn , esser;t ially U n i s h e s as the i w e r s e square of t he dis tame. Like- I wise, the s t rength of the re f lec ted signal diminishes i n a similar manner i n

returning t o Earth. proportional t o the inverse fourth power of the a l t i t ude . Hence, a l t i t udes of several thousand miles are about the upper l i m i t . these l o w a l t i t udes do not remain i n best s igna l r e f l ec t ing posit ion f o r very long, s o many s a t e l l i t e s would have t o be employed t o maintain continuous communication between any two s ta t ions .

Thus, the s igna l strength at the receiving s t a t i o n is

Individual s a t e l l i t e s a t I

Active s a t e l l i t e s , such as T e l s t a r and Relay, where the s igna l i s re - ceived and amplified before being transmitted t o the ground, can be used a t much higher a l t i t udes . A t an a l t i t u d e of 22,300 miles, t h e s a t e l l i t e would remain nearly s ta t ionary i n the slqy because i t s period would coincide w i t h that of the Earth's rotat ion. type. hemisphere. small region around the poles.

Syncom ( s l ide 8) i s the first satell i te of this A single synchronous s a t e l l i t e can provide communication f o r nearly a

Three such s a t e l l i t e s could cover the e n t i r e Earth except f o r a

Have you ever noticed how much l igh t there i s i n the sky on a c lear moonless night out i n the country? stars and planets. "air glow" or iginates at an a l t i t u d e of perhaps 90 t o 100 kilometers. caused i n i t i a l l y by a t r i p l e co l l i s ion of t h ree owgen atoms. One of these at this a l t i t u d e r e t a ins electrons i n an elevated energy leve l . Some time later, t h i s forbidden neut ra l oxygen atom releases i t s energy a t a wavelength of 5577 angstrom units t o give the " a i r glow." f i l t e r that would pass only l i g h t of 5577 2 10 angstrams. s t i l l v i s i b l e t o Carpenter through this f i l t e r showing the cause conclusively.

Such l i g h t surely does not come f r m t h e

It i s Even the Milky Way i s very spec i f i ca l ly located. This

Carpenter car r ied along a spec ia l The a i r glow w a s

This air glow i s by far the best horizon f o r the astronauts. It, of course, l ies agove the Earth 's horizon. Stars can be seen between the a i r glow and the Earth. horizon qui te accurately. He did this by noting the exact time when individual known stars just entered and lef t the air-glow region. posi t ion of the Mercury capsule were known, the rest of t he computation follows i n a straight-forward manner.

Carpenter w a s able t o determine the a l t i t u d e of this air-gluw

Since the a l t i t u d e and

Ft e l s e can the astronaut see f rm a s a t e l l i t e ? t h e heavens unhampered by the turbulence of the atmosphere. therefore not t w i n k l e .

He cer ta in ly can see Stars would

He can get a marvelous view of the Earth 's cloud cover and weather patterns; perhaps he can even detect hurricanes i n t h e i r formation before ground-based s t a t ions can do so (slide 9.) cover p ic ture that has been obtained, but I like it because I believe that it

This is far from the best -cloud

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represents the first photograph of a hurricane i n the making Oaken from a rocket.

The Tiros s a t e l l i t e s , with much more beaut i ful pictures, have demonstrated the a b i l i t y t o observe and improve the mapping of world-wide weather patterns. These s a t e l l i t e s have been used t o determine the center of a t rop ica l storm, t h a t was i n error by 500 kilometers by conventional methods; the break i n an Australian heat wave was accurately predicted from Tiros I1 data; and Hurricane Esther was discovered by Ti ros 111, e tc . from more sophisticated s a t e l l i t e s such as Nimbus and s t i l l later, Aeros.

These w i l l be followed by contributions

These gross patterns w i l l surely be v is ib le t o an astronaut. well can he detect details and Ear th surface charac te r i s t ics? The answer t o this question comes from simple physics.

But how

The quality of t h e image from an opt ical o r radar viewing system i s

The circular aperture ( o r any other shaped opening) on such a viewing l imited by the f a c t t h a t electromagnetic radiations have wavelike character- i s t i c s . system w i l l generate a d i f f rac t ion pat tern so t h a t a point source w i l l not give a point image on the viewing screen. The individual l i g h t i n t ens i t i e s from the images of two point sources are p lo t ted on s l i d e 10.

Clearly, i f the two images are closely spaced, the l i g h t patterns w i l l blend s o that they cannot be distinguished as separate. Conventionally, t h i s closest spacing f o r resolution occurs when the cent ra l max imum of one point source f a l l s a t t h e f i rs t minimum of the second point source. This gives a minimum angular resolution of

Qi = 1.22 A a

where

h i s the wavelength of the radiation

a i s the aperture diameter

0

This angle (slide 11) i s a l so equal t o the distance between the two point sources x divided by the distance d of the two sources from the observation stat ion.

i s the angular separation of t he two point sources, i n radians

Hence,

the quantity x viewed object.

approximates the uncertainty of posi t ion o r def in i t ion of a

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Using equation (9 ) , a 1-inch diameter op t ica l telescope mounted on a s a t e l l i t e 200 miles above the Earth canbe used t o resolve point l i g h t sources

distance i s 1.9 feet. fuzziness of the boundaries of the object under observation. Thus w i t h a 12-inch scope, ships, roads, buildings, t r a ins , and general map charac te r i s t ics , including automobiles i n parking l o t s , could be determined. A satellite equipped with a reasonable telescope (=-in. objective) could probably detect , c lass i fy , and es tab l i sh the location of a l l o w surface ships as well as m i l i - tary ins t a l l a t ions and troop mwements. of a l l the Russian ships without the a i d of espionage and code breaking.

I if they are greater than 23 f e e t apart. With a 12-inch scope, the resolved This resolved distance approximately represents the

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We, i n turn , , could know the locat ion

The question might be ra i sed as t o whether there is su f f i c i en t i l l u m i - nation f o r observation. The l i g h t transmission coeff ic ient through the er;tire atmosphere i s about 85 percent a t the zenith so that the Earth would surely appear t o be br ighter than the f u l l Moon (ac tua l ly by six or seven times). Viewing the zeni th through the e n t i r e atmosphere i s about equivalent t o looking at an object 5.3 miles away on the surface. much grea te r from the Earth than from the Moon because of r e l a t i v e s izes and distances.

During the daytime, there ce r t a in ly is.

The v i s ib l e l igh t available t o a satel l i te observer would be

How small a l i g h t could one see on the ground? According t o H.N. Russell, Astrophy J. 45, 60 (19179, the unaided human eye requires a t least 2.5>0_W9 ergs per second of energy t o detect a point l igh t source. a 1-percent efficiency should, therefore, be observable from a s a t e l l i t e located a t an a l t i t ude of 200 miles i f a 12-inch telescope objective were used. A photographic p la te might require a 60-watt l i g h t bulb t o be v i s i b l e f o r a 1-minute exposure time. a manned s a t e l l i t e even at night.

A 1 - w a t t l ight bulb with

Thus, useful observations of E a r t h could be made with

You a l l know, of course, that the stars appear t o twinkle w h i l e the

temperature gradients. These disturbances modify the index of re f rac t ion of the atmosphere and cause l o c a l bending of the l i g h t rays passing through it. Thus, t he posi t ion of a star appears t o change t r ans i en t ly w i t h time, which leads t o the twinkling sensation. atmosphere is s t a t i s t i c h l l y uncertain by 1 t o a few seconds of arc. t he angular s i ze of the planets i s larger, the twinkling is not apparent, even though it i s s t i l l there . of about 17 sec a t c losest approach. Mars disc , or on the Moon f o r that matter, would have the same c i r c l e of confusion due t o turbulence as does a star).

I planets do not. This t w i n k l i n g e f f e c t i s due t o atmospheric turbulence and

The position of a star as seen through the Because

(Mars, f o r example, subtends an angle from Earth A pinpointed object on the rim of the

The pos i t iona l uncertainty of a s a t e l l i t e as viewed from the ground might be as much as 3 seconds of a r c associated w i t h atmospheric turbulence. The corresponding distance e r ro r f o r a s a t e l l i t e a t 200 miles i s about 15 fee t . I t o l d you e a r l i e r , however, that a s a t e l l i t e observer w i t h a 12-inch lens could see the ground within an accuracy of 2 f ee t . Maybe I can c l a r i f y this d i f f i c u l t y wi th slide 12.

You may ask, "How come?"

10 I .

You see, the index of refract ion of a gas i s re la ted t o i;ts density. But the density of t he atmosphere from equation (5) decreases exponentially with a l t i tude . Hence, the air layers near t he surface of t he Earth (where the density of the a i r i s high) produce the greatest bends on the l i g h t path. s l i d e 12 , the same l i g h t path i s t ravel ing between the ground observer and the s a t e l l i t e astronaut, but each appears t o see h is object along the projected tangent t o the loca l l i g h t path. while hardly any bending occurs near the satel l i te . the observational e r ror of the astronaut i n observing the ground i s much l e s s than tha t of the ground observer viewing the s a t e l l i t e . I n f ac t , the r a t i o of the e r rors can be shown t o be about 1 t o 45 f o r a 200 mile a l t i t ude s a t e l l i t e . Thus, atmospheric shimmer causes an e r ror of only a few inches i n the astronaut 's observation of the ground. An up t o 6 ' diameter objective could be used on a s a t e l l i t e a t 200 miles f o r viewing the ground before the inherent op t ica l resolution was b e t t e r than the resolution l i m i t due t o turbulence and atmospheric shimmer.

On

The bending is great near the ground observer Hence, you can see that

Having raised the question as t o how well the astronaut can view the ground, it might be in te res t ing t o spend a f e w minutes discussing orb i t s ( s l i d e If the s a t e l l i t e i s lauhched from e i the r pole, only a polar orb i t can be established. O f course, the en t i r e Earth would come under surveil lance as the Earth rotated under the s a t e l l i t ' s o rb i t . Clearly, however, it i s not possible t o es tabl ish an equator ia l o rb i t with a b a l l i s t i c launch from the poles. f ac t , the incl inat ion of the o r b i t a l plane t o the equator must be equal t o or greater than the la t i tude of the launching s i te . Thus, only from the equator can a l l kinds of orbi ts , t h a t i s , equatorial , polar, etc. , be established

I n

Now, the posit ion of the o rb i t a l plane of t he s a t e l l i t e depends on the inhomogeneities of the Earth 's g rav i ta t iona l f i e l d . The Earth i s not a perfect sphere, s o that the idealized var ia t ion of the grav i ta t iona l a t t r ac t ion w i t h the inverse square l a w holds t rue only approximately ( s l i de 14) .

The actual Earth bulges a t the equator. The var ia t ion i n gravi ty some- w h a t resembles t h a t of a spherical Earth with a b e l t around the equator. gravi ta t ional p u l l of this b e l t is stronger on a sa te l l i t e a t the equator than a t the poles. The Earth 's bulge thus influences the orb i t of a sa te l l i t e . The perturbations of t h i s orb i t may, i n turn, be used t o make geophysicalmeasure- ments on the Earth's gravity. Such s tudies with Vanguard I lead t o the dis- covery of the pear-shaped Earth.

The

There are two important ways i n which the equator ia l bulge influences the s a t e l l i t e ' s o rb i t . The f i rs t i s t o def lect t he sa te l l i t e toward the normal t o the equator each t i m e it crosses. Thus, t he plane of an eastwardly launched s a t e l l i t e w i l l r o t a t e toward the west a t a rate i n degrees per day of about

R = 8 COS u

where a i s the incl inat ion of the o r b i t a l plane t o t h e equator. Also, t he s a t e l l i t e speeds up i n crossing the equator became of t he bulge. This Causes

13).

. . ll . .

the major a x i s of the e l l i p t i c a l path t o ro t a t e i n the plane of t he orb i t a t a rate i n degrees per day of about

s = 4(5 cos2 a - 1) (ll) '

The direct ion of this ro ta t ion reverses above l a t i t udes of about 63'. about the plane angle of the ear ly Russian satellites. Thus, they could keep the perigee over Russia, which would enhance the data transmission t o Russians. The numbers i n equations (10) and (u) are f o r a 200-mile-altitude s a t e l l i t e .

This is

You may note that the satell i te i s always f a l l i n g i n orb i t tward the Because of the Earth 's curvature, however, the as t ronaut ' s horizon Earth.

keeps dropping i n f ront of h i m so that he does not reach the surface but continues t o go round and round. then experiences the phenomena associated with weightlessness.

A body i n free fa l l , such as the s a t e l l i t e ,

The weightless environment i s s t i l l of considerable worry t o the space The influence of weightlessness on man f o r extended periods of s c i en t i s t s .

time might lead t o deteriorations of m u s c u l a r and body functions through lack of stimulation. should s t ay on the top, the bottom, o r the sides of the tank. Hence, venting of the tank may be as much of a problem as locating the f u e l a t the tank discharge port.

The f u e l i n a rocket tank may be undecided as t o whether it

Fortunately, forces normally neglected at 1 g can be u t i l i z e d at zero g t o help us. Standpipes can a l so be in s t a l l ed t o locate the l iqu id near the discharge ports.

If the f lu id wets the tank w a l l , t he l i qu id w i l l c l ing t o the w a l l s .

Gravity Zero gravi ty

"his i s a l l I w i l l say about zero "g" environment but it remains of continuing research i n t e r e s t and concern.

The thermal environment i n space depends strongly on where one chooses t o be i n space. Space has such a high vacuum - of the order of meters of mercury or be t t e r - t h a t only a few molecules of hydrogen are present i n each cubic centimeter. depends upon a s t a t i s t i c a l d i s t r ibu t ion of molecular or vibrat ional speeds probably has no meaning. m u s t be pr incipal ly by radiation.

m i l l i -

Hence, the normal def ini t ion of temperature that

A l s o , t he heat t ransfer t o or away from a spacecraft

One might then define the tem-perature of space t o be that equilibrium temperature that a body would assume i n the absence of sunlight or planet l i gh t .

12

This temperature f o r deep space i s perhaps 3' t o 4' K but may-be as high as 20' K i n portions of the M i l k y Way. i s therr determined by equating the energy absorption by the body from the so lar and planet radiations t o the energy reradiated t o deep space and t o any nearby objects. of the absolute temperatures.

The temperature of a body i n so l a r space

The radiat ion from a black body i s proportional t o the fourth power

Q = k(T1 4 - Tt)

Now, the energy received near the Earth from the Sun i s 1.34 kilowatts per square meter. This energy i s largely i n the v is ib le range. If it i s absorbed by a spacecraft, the reradiat ion i s largely i n the infrared range, but materials have different absorption and emissivity coeff ic ients according t o the wavelength of the radiation. Hence, the temperature of spacecraft may be controlled by select ion of appropriate surface coatings. A coating with hig absorptivity i n the v is ib le region and low emissivity i n the infrared region w i l l have a higher equilibrium temperature than i f the reverse i s t rue . By cmbinations of s t r ipes and selected coatings, spacecraft temperatures a re usually adjusted t o be near normal room temperatures. The coatings are p re t ty sophisticated, being. adjusted f o r par t icu lar o r b i t a l paths or missions. Mariner, f o r example, would have a d i f fe ren t coating than satell i tes near Earth. path i s changed because of delays of a few days a t launch.

Even on near-Earth s a t e l l i t e s , the coatings are a l t e r ed i f the o r b i t a l

A simple energy balance w i l l indicate the var ia t ions of equilibrium temperature on a spacecraft i n so la r space. suff ic ient conductivity t o have uniform surface temperature, the black body so lar energy absorbed w i l l vary inversely as the square of the distance f rm the sun. The energy radiated w i l l be 'proport ional t o the fourth power of the surface temperature. The equilibrium temperature obtained by equating the absorbed and radiated energy thus varies inversely as the square root of the r ad ia l distance from the Sun.

If we assume a sphere with

The Ear th , as you a l l know, has a magnetic f i e ld . important influence on the near-Earth space environment. t o show you b r i e f ly w h y t h i s i s the case. simplified discussion of magnetohydrodynamics.

This f i e l d has an I would l i k e now

I hope you won't mind a very

A charged par t ic le , such as an electron o r a posi t ive ion, feels a force if it i s placed i n an e l e c t r i c f i e l d . q and the f i e l d strength E.

This force i s proportional t o the charge

+ + :'o +

F = qE

13

I n t h e i l l u s t r a t ion , the f i e l d is established by the charges on the plates of a condenser. f i e l d .

Clearly, a posi t ive charge i s forced i n the direct ion of the

Also, when an e l e c t r i c current flows i n a wire placed i n a magnetic f i e l d , there i s a force on that wire that i s proportional t o both the f i e l d s t rength and the current and i s perpendicular t o both. A magnetic f i e l d p a r a l l e l t o a current -carrying element produces no force.

Current i

I n vector notation

i out of plane of Paper

Now, t h e current have a single charge

i i s the r a t e a t which charge is passing a point. If we q passing through our "conductur," the current is

- - i = a = q G at

Hence, t he force on a moving charge due t o the magnetic f i e l d is

F = q ; ; x i i

Thus, if w e combine equations (16 and (1311, the force on a charged pa r t i c l e

14

moving i n e l ec t r i c and magnetic f i e lds i s

where consistent un i t s must be used.

You w i l l note i n equation (16) that the force due t o a magnetic f i e l d i s always normal t o the velocity. nor subtracts energy from the p a r t i c l e ' s motion. par t ic le i n a constant magnetic f i e l d follows a circular path around the f i e l d l i n e such tha t the magnetic force balances the centrifugal force:

Hence, a steady magnetic f i e l d neither adds Thus, a moving charged

l o r the path radius i s

Thus, charged par t ic les are trapped by a magnetic f i e l d .

I f we now superimpose a force normal t o the magnetic f i e l d l ines , f o r example, by e i ther an e l e c t r i c f i e l d o r a grav i ta t iona l f i e l d , the par t ic les w i l l be a l ternately accelerated and decelerated by t h i s new force. motion of the par t ic le i s t h a t shown on s l i d e 15. This motion i s the super- posit ion of a constant drift velocity

Hence, t he

- Exi r i w = - H2

and the circular o r b i t a l velocity without t he e l e c t r i c f i e l d . t h a t the d r i f t velocity i s independent of the charge, so t h a t both posi t ive and negative charges migrate i n the same direct ion even though t h e i r o r b i t a l ve loc i t ies are \opposite. i n a direction normal t o the magnetic and gravi ta t iona l f i e l d s .

You w i l l note

The Earth 's g rav i ta t iona l f i e l d produces a similar type motion

Now, a velocity component pa ra l l e l t o t he magnetic f i e l d produces no force; hence, charged par t ic les can follow s p i r a l paths along the magnetic f i e l d l ines , l i ke t h i s :

H

15 ..

then However, i f the f ie ld is increasing i n the direct ion of the s p i r a l motion, the radius of the path w i l l decrease, and the converging f i e l d l i n e s w i l l

produce a force t o r e f l e c t the pa r t i c l e back i n the direct ion whence it came. Thus, we could make magnetic mirrors t o t r a p charged pa r t i c l e s i n the region between increasing magnetic fields:

F ie ld l i nes

Distance

We can now t a l k about the V a n Allen belts ( s l i d e 16). You see, t he Earth has a magnetic f i e l d surrounding it. poles. pa r t i c l e s . bouncing back and f o r t h along s p i r a l paths from pole t o magnetic pole. Simultaneously, there i s a very small drift veloci ty i n the circumferential d i rec t ion associated with the gravi ta t ional f i e ld . diffuse around the Earth t o form the Van Allen be l t s .

This f i e l d increases toward the Hence, t he Earth 's magnetism forms a magnetic b o t t l e t o t r a p charged

These pa r t i c l e s a re spiral ing around the magnetic f i e l d l i nes ,

Thus, the charged pa r t i c l e s

"he Van Allen b e l t s generally do not pe r s i s t t o the lower altitudes of

The belts a re strongest i n the regions between 1 and 10 Project Mercury space f l i g h t s . cause t h e i r decay. Earth radii.

If they did, the upper atmosphere would soon

Data from Explorer M I ( s l i d e 1 7 ) reversed previous conceptions of the be l t s . ins tead of two d i s t i n c t be l t s . Earth 's magnetic f i e l d .

The e n t i r e region i s ac tua l ly a single system of charged pa r t i c l e s These charged pa r t i c l e s a re trapped by the

"he chief consti tuents of the Van Allen b e l t s a r e electrons and slow The number of each kind var ies from a l t i t u d e t o a l t i t u d e moving protons.

( s l i d e 18.) of tens of mill ions of e lectron vol ts (10 M e V ) .

A t 2000 miles, the predominant pa r t i c l e s were protons with energies This region has been modified

. , 16

by nuclear explosions. A t 8000 m i l e s , protons with only a f rac t ion .of an MeV predominate, and a t 12,000 miles, protons with energies of 0 . 1 t o 4 MeV and electrons with energies up t o 2 MeV are blended. The exact so1.rce of t he charged par t ic les i n these regions i s s t i l l a matter of controversy.

The formation of one new a r t i f i c i a l be l t , however, i s well understood. I n t h i s one, a high-altitude nuclear explosion f i l l e d the region with electrons. The surpr ise associated with t h i s one i s tha t t he s t rength of the b e l t i s pers is t ing much longer than or iginal ly estimated. Current speculations suggest t h a t the be l t may be detectable f o r as long as a year. has knocked out the power supplies of three s a t e l l i t e s . This degradation is due t o damage t o the so la r ce l l s .

I n the meantime, it

The outer regions of the Van Allen b e l t s contain large numbers of low- energy protons. These protons pose less of a radiat ion hazard than high-energy electrons. Man, passing quickly through these regions on the way t o the Moon o r beyond, would be i n l i t t l e danger from the protons but would need t o be protected f rom the X-rays generated by impingement of the electrons on the spacecraft components. On the other hand, the protons would pose a serious problem even f o r a heavily shielded vehicle t o the occupants of an e l e c t r i c propulsion spacecraft or an orbi t ing laboratory i f the residence time approached two weeks or longer.

Let me define some terms dealing with radiat ion exposure. The most common uni t for a radiat ion dose i s the rem or Roentgen equivalent man. r e m i s t h a t quantity of any type of ionizing radiat ion t h a t , when absorbed i n the human body, produces an e f f ec t equivalent t o the absorption of 1 roentgen of X o r gamma radiat ion a t a given energy. X o r gamma radiation required t o produce i n 1 cubic centimeter of dry a i r ions carrying 1 elec t ros ta t ic u n i t of posit ive or negative charge. Now, fo r a two-week exposure time i n the inner Van Allen b e l t with a 25-rem exposure l i m i t , the shield weight would have t o be about 140 grams per square centimeter (55 in . of water thickness).

The

The Roentgen i s the quantity of

I don't want t o leave you with the impression that the Van Allen be l t s a r e steady and unchanging. explosions. During so la r f l a r e s and intense sunspot ac t iv i ty , tongues of plasma ( s l i d e 1 9 ) along with a trapped so lar magnetic f i e l d a re shot out toward t h e Earth. geomagnetic sphere of the Earth forms a shock wave i n t h i s plasma sheath; consequent distortions of the Van Allen b e l t s occw as pictured i n s l i d e 20. The edge of the geomagnetic sphere confined inside t h i s shock wave and on the s ide of the Earth facing the Sun i s at an a l t i t u d e of some 30,000 t o 40,000 miles.

We have seen how they have been modified by nuclear They a l s o a re modified and d is tor ted by the so l a r winds.

The

We should now discuss b r i e f ly the space rad ia t ion hazard t o man. background normal cosmic ray in tens i ty from a l l sowces i s about 0.65 rem per week. t rave ler would reach h is l i m i t on radiat ion exposure from t h i s source alone i n about 38 weeks.

The

Thus, with a rather high 25-Pem exposwe l imi t , an unshielded space

The composition of these cosmic rays i s shown on s l i d e 21.

17

Much more serious a re the cosmic ray burs t s f romthe Sun. These pa r t i c l e s are known t o be mostly protons w i t h energies ranging from l e s s than 10 MeV t o almost 50 Bev.

The c l a s s i f i ca t ion df these solar flares i s shown i n slide 22. The minor C l a s s I and I1 flares are not par t icular ly troublesome. f l a r e s are more serious, with 60 such events recorded between January 1956 and 1961. giant so la r f l a r e s la rger than III+ have occurred. i n the past 18 years.

The Class III and III+

Hence, major f l a r e s might on the average occur monthly. Seven bave been observed

Occasionally,

Weight l imitat ions may r e s t r i c t the Apollo shield t o between 10 and 30 grams per square centimeter. This re la t ive ly t h i n sh i e ld w i l l protect the man for the short times that he might spend i n the Van Allen belts and against minor and major so l a r flares. His greatest danger could come from unpredicted giant so l a r f l a r e s .

Almost a l l protons of energies greater than 100 t o 200 MeV w i l l pass

protons per square centimeter per through a 10- t o 30-gram-per-square-ce timeter shield. prminent a t flux leve ls as high as 10 second f o r a Class III+ solar flare. i n s l i d e 23.

These energies a re f The d is t r ibu t ion of the f l u x is presented

The energy loss mechanism (ionization and s t raggl ing) i n t h i n shields i s well behaved and understood. I n t h i n shields, t he protons w i l l col l ide only occasionally with a sh ie ld nucleus, and hence secondary radiat ions a re unim- portant. as is shown i n s l i d e 24.

The penetration of protons i n carbon and aluminum i s straightforward

Future space f l i g h t s of much longer duration, however, w i l l require th ick shields. i n the rad ia t ion shield and the spacecraft mater ia l become important. of these reactions a re shown i n the next s l i d e (sl ide 25). reaction, a proton en ters a heavy nucleus, which then dis integrates t o a l i g h t e r nucleus with emission of protons and neutrons. other dis integrat ions. t o form a radioactive atom. cause fu r the r reactions.

Then the secondary radiations a r i s ing from nuclear reactions Some

I n the cascade

These, i n turn, cause I n the evaporation reaction, a proton en ters a nucleus

The l a t t e r might decay emitting a neutron t o

The e f f ec t of these secondary radiations is estimated i n s l i d e 26 f o r the May 10, 1959 f l a r e . cesses and rad ia t ion interact ions needed f o r predicting the effectiveness of t h i ck shields a re understood cnly poorly. the energy imparted t o matter by ionizing rad ia t ion and i s equal t o 100 ergs per gram of material . creasingly more important r e l a t ive t o the primary ones as the sh ie ld thickness increases.

I would l i k e t o caution you, however, that the many pro-

The "rad" shown is a measure of

You can see that the secondary radiat ions become in-

The danger from gian t so l a r f l a r e s r e s u l t s pa r t ly f romthe i n a b i l i t y of Earth s c i e n t i s t s t o predict them. ea l c -da tkg the s i z e of the dark-shaded area (penumbra) surrounding each

Some success has been obtained by first

18 .. sunspot group ( s l ide 4). Then the number of groups (flares) with penumbral area greater than 300 millionths of the solar surface, 500 mill ionths, 1000 mill ionths, etc. , i s calculated. whether one could expect a safe f l i g h t or not f o r a 4- or 7-day period corresponding t o Apollo plans. of the Goddard Space Fl ight Center showed the following:

These numbers a re then used t o predict

Examination of the r e su l t s by Harriet Malitson -

(1) A cr i te r ion which require penumbral areas la rger than 1000 mill ionths of the so la r surface f o r unsafe f l i g h t reduces the usable f l i g h t time by 33 t o 40 percent and s t i l l allows encounters with about half the events. This i s not good enough!

( 2 ) Reduction of the area t o 500 mill ionths of the so la r surface takes care of a l l but one of the events f o r 1949 and 1950 but cuts down the "safe" time t o 20 t o 35 percent of the ac tua l usable time. For the year 1951, even reduction of the exclusion area t o 300 mill ionths of the so l a r surface leaves two encounters.

I wish t o leave this subject with the thought t h a t we do not have adequate methods f o r predicting the large so la r f l a r e s . would be dangerous t o extended-time manned mission f l i g h t s t o Mars, e tc .

Hence, t h i s radiat ion source

The next environmental fac tor I would l i k e t o discuss i s t h a t of meteoroids. A meteroroid i s any of the countless small bodies i n the so la r system. incandescence through the atmosphere, it i s a meteor; and i f it reaches the surface of the Earth, then it becomes a meteorite.

Let 's s t a r t with some def ini t ions. I f the meteoroid passes with

Now, the meteoroids vary i n both s i ze and density. be as l i g h t as snow with a spec i f ic gravi ty of perhaps 0.15. meteorites seen i n museums have spec i f ic grav i t ies of 2 t o 3, while the spec i f ic grav i t ies of the nLckel-iron ones are more l i k e 7 or 8. The s izes of t he meteoroids range from inf ini tes imal up t o perhaps a f e w pounds o r heavier. You a l l have heard of the meteor c ra t e r i n Arizona t h a t i s about a mile across. Meteors of suff ic ient s i ze t o produce such a change i n the Ear th ' s s t ruc ture must be large indeed.

Some of them may The stony

The meteoroid mass-frequency d is t r ibu t ion estimated by several observers i s shown i n slide 27. meteor t ra i ls . a t ion i n the atmosphere, the r a t i o m/C$ may be estimated. the equation of motion fo r a ve r t i c l e meteor i s

These curves a re generally obtained by observers of By following the meteor and measuring i t s speed and deceler-

For example,

f .

19

2 The acceleration d y/dt2 and the speed v a re measured. The a i r density P i s estimated from previous sonde data. obtained.

Hence, an estimate of m/C+ i s Now, reasonable guesses of the d r a g coeff ic ients can be made.

We may a l so measure the brightness of the meteor, and brightness is r e l a t ed t o temperature and rate of evaporation of material fromthe surface. Hency, an estimate of the mass of the meteroid can be obtained. I n this way, so crudely outlined, the curves shown a re obtained (sl ide 27). The possible e r rors due t o the assumed ef f ic ienc ies of the various processes can be large.

A r t i f i c i a l meteors have been produced i n Project Trailblazer. ( Ikrvard). Here, t he mass and the density of t he a r t i f i c i a l meteor were known pr ior t o the launch of the rockets; hence, a cal ibrat ion of the method resu l t s . This ca l ibra t ion suggests that some of the previous flux estimates were high by perhaps an order of magnitude, especially f o r t he la rger s ized pa r t i c l e s ( s l i de 27). of the ionized meteor trails i n the upper atmosphere. see on the s l ide , s a t e l l i t e data give some hazy confirmation. The e a r l i e r sa te l l i te data shown here a re generally obtained by recording impacts on a p la t e by means of a microphone. The f lux i n these experiments is f a i r l y good, but there i s considerable uncertainty of the mass. People are not sure whether the conversion of meteoroid speed t o noise i n t h e microphone i s an energy- or a momentum-dependent process; and t h e question depends on the speed and d i rec t ion r e l a t i v e t o the impac-k plate.

The data on this curve also receive support from radar observations Likewise, as you can

The next natural question is, "How are these pa r t i c l e s d i s t r ibu ted i n space?" are many more pa r t i c l e s h i t t i n g the Earth on t h e leading hemisphere than on the t r a i l i n g hemisphere as the Earth moves i n o rb i t around the Sun. data a re corrected f o r the speed of the Earth, then the opposite is t rue. is, the majority of the pa r t i c l e s a r e t ravel ing i n orb i t s , presumably around the Sun i n the same general direct ion as the Earth.

This is shown i n slide 28 i n two ways. To an Earth observer, there

If these That

The d is t r ibu t ion of the par t ic les r e l a t ive t o t h e plane of the e c l i p t i c i s shown i n slide 29. about 20' on each s ide of t he o r b i t a l plane of the Earth about the Sun. of them are orbi t ing around the Sun, but some of them may a l so be trapped i n o rb i t s around our Earth-Moon system. the plane of the e c l i p t i c and are probably of cometary origin. guessing that these have very l o w densities - less than 0.5 gram per cubic centimeter.

You can c lear ly see that most of the par t ic les are within Most

Some of the pa r t i c l e s make large angles t o W e a re

Le t us speculate a s t o the origin of the meteori t ic material. It might come from outside the so la r system. t o pass on through and t o lease the region again on hyperbolic-type o rb i t s unless it suffered a gravi ty tu rn near one of t he planets which would lead t o trapping.

If so, one would expect it generally

A s an i l l u s t r a t i o n of a gravi ty turn that may increase or decrease the

Relative t o the planet, the path w i l l be a hyperbola energy of the par t ic le , consider t he path of a pa r t i c l e that makes a neak approach t o a planet.

20 a .

(sketches A1 and B1) with the same speed 'nut with a different -direction a f t e r the near miss.

Sketch A1 (Planet adds energy)

: Planet

Sketch B, o/ L

(Planet subtracts energy)

This means that , a f t e r the near encounter, the speed w i l l be d i f fe ren t r e l a t ive t o a fixed point i n so la r space. the gravi ty turn adds or subtracts energy t o the meteoroid a t the expense of the planet.

"he vector diagrams A2 and B2 i l l u s t r a t e how

Solar veloci ty

\ \ Solar

veloci ty Solar before ,Ay \ after encounter encounter \

\ veloci ty

\

encounter

Lve loc i ty r e l a t ive t o planet before encounter

Sketch A2 ( Planet adds energy)

p L Velocity r e l a t i v e t o planet before t o planet a f t e r encounter

Velocity r e l a t ive

encounter

Sketch B2 (Pla.net subtracts energy)

- . . .

21

The meteorbids might come from the asteroid belt between Mars and Jupi ter . gravi ty turns, The o rb i t s of the individual pa r t i c l e s can be a l t e r ed upon 'close approach t o another mass center. surely the meteoroid population near Mars would be la rger than near Earth.

You see, pa r t i c l e s can diffuse out of this b e l t by a combination of

If this explains meteoroids, then

The meteoroids near Earth might actual ly come from the Moon. You see, a meteoroid impact on the surface of t h e Moon may carry enough energy so t h a t four or f i v e times as much mass would be knocked off the Moon's surface t o escape veloci ty as that of the impinging meteoroid. who be l ieve ' tha t the t e k t i t e s found on the Ear th cane from the Moon.

There are eminent s c i e n t i s t s

Then too, meteoroids may originate in the tai ls of comets. This theory i s in te res t ing but does not explain where the comet came from.

Why are w e s o in te res ted i n meteoroids? The pr inc ipa l reason is because there is a f i n i t e probabi l i ty t h a t the in t eg r i ty of space vehicles might be destroyed by meteoroid puncture. W e must make the various cmponents su f f i c i en t ly th ick t o prevent penetrations or destructive damage. Slide 30 gives some indication of the number of punctures t o be expected i n stainless steel. This chart i s based upon Explorer XVI: data obtained near Earth. N e a r Mars, the meteoroid population might be a f ac to r of 2000 larger , but w e don't know f o r sure.

The uncertainty of the information on both meteoroid f l u x and penetration You s a w how one experiment, the l'Trailblazer," changed the c r i t e r i a is great.

f lux estimates. The problems here are multiple. density of the par t ic les . average speed range of the meteoroid par t ic les . They range from ll t o 73 k i lo- meters per second. Hence, we urgently need t o launch large-area meteoroid damage experiments i n space t o determine survival probabi l i t ies . Without this information, spacecraft w i l l l i k e l y be e i t h e r underdesigned leading t o ear ly loss of mission, or averdesigned, giving overweight or sluggish performance. Either s i t ua t ion is expensive. I n f a c t , the whole future of e l e c t r i c propulsion may depend upon a c l a r i f i c a t i o n of the meteoroid survival probabi l i t i es f o r the radiator . Our present estimates of the weight of the system may be i n e r ro r by as much as a f a c t o r of 3. This weight difference would make a tremendous difference i n the performance of an e l e c t r i c a l l y propelled spacecraft.

A corresponding uncertainty ex i s t s i n the penetration c r i t e r i a . W e do not have a very good f e e l f o r the

And we don't have any ground'based data i n the

The fastest gun barely approaches the lower f igure.

Before closing, I would l i k e t o discuss very b r i e f l y the planetary environments f o r manned exploration. The i n t e r e s t i n manned f l i g h t s t o Venus cooled suddenly when Mariner I1 data suggested a surface temperature of 8000 F. Since la ter spectra (John Strong) showing the presence of water vapor have cas t some question on this value, same in t e re s t i s being revived. We need --- mn-e i n c f n m f p n f d ---- -- prnhc r3Rt.R tm decide whether manned Landings on Venus would be feasible.

. . 22

1 .

Mars has proverbially atkracted the imagination as a planet f o r potent ia l human development. However, on careful study, the environment on the Martian surface is anything but a t t rac t ive . The surface pressure, f o r example, i s about 25 millibars corresponding t o about 85,000 fee t a l t i t ude on Earth. The oxygen content i s l e s s than 1 percent corresponding t o about 155,000 f e e t a l t i t ude on Earth. moisture on Earth s o that r ivers , lakes, and oceans would probably be non- exis tent .

The water vapor content i s perhaps only 1/2000th of the

What l i t t l e water there i s might be sa l ty . Mars would a l so l i k e l y have i t s surface bombarded with asteroids and meteoroids from the as te ro ida l be l t . This could be much more serious than on Earth because of the greater meteoroid population near Mars, and because of the t h i n atmosphere. On Earth, meteors penetrate t o perhaps 50,000 feet a l t i tude . A similar pa r t i c l e on Mars would s t r i k e the surface. Also, the theory of planet formation suggests that Mars would have a so l id core and no mountains of the folded type. core, there probably i s no appreciable magnetic f ie ld , and hence, no trapped radiat ion be l t s of the Van Allen type. On the surface of Mars, severe radiat ion in tens i t ies would be expected during giant so la r flares because of both the rarefied atmosphere and the lack of magnetic f i e l d trapping of ionized par t ic les . surface during giant so la r f l a r e s .

With a s o l i d

Thus, strong shielding would be required on the Martian

To survive, man would have t o take h i s Earth environment with h i m .

Nevertheless, manned exploration of Mars w i l l take place and can Conditions a r e so hos t i le t h a t colonization i s probably out of the question i n our times. be just i f ied solely on i t s sc i en t i f i c merits.

Figure 1

? N N rn w I

H,

Figure 3

Figure 4

.

4 -1

I Q LD N

' N I w

ATMOSPHERIC COMPOSITION

HYDROGEN

Figure 5 C 5 -26322

REFRACTION OF SKY WAVE

NOTE THE SKIP Z O N E

f

. .

THE MESSAGE RELAY SATELLITE .

TE H H

1. TRANSMITTED AT A, RECORDED 2. RETRANSMITTED, RECEIVED AT B 3. RELAYED AT POLE TO OTHER

SATELLITES

Figure 7

Figure 8

ROCKET VIEW FROM 100 MILES ALTITUDE

.. . ~~

Figure 9

RESOLVING POWER FOR CIRCULAR APERTURE

x 'p = 1.22 a

A = WAVE LENGTH

a = DlAM OF APERTURE

9' MINIMUM ANGULAR RESOLUTION I

CS- 16082 Figure 10

LENS, OlAM a,

I N. OBJECT

X

FOR A = 4500 xa = 23 FT

CS-16085 0 I., X 23 FT; 0 = 12', x = 1.9 FT

Figure 11

ATMOSPHERIC SHIMMER LESS SERIOUS TO SATELLITE OBSERVER

APPARENT

CS-16079 AX 5 - 4 5 A%

Figure 12

SATELLITE ORBITS

H H M LD N N

I 1 I W

POLAR ORBIT

CONSTANT LATITUDE

POLAR ORBIT

CONSTANT LATITUDE

ORBIT

CS-15754

LEOUATORIAL PLANE Figure 13

EFFECT OF EARTH ON SATELLITE MOTIONS

MAJOR AXIS MOVES EASTWARD FOR a < 63"

N

ORBITAL PLANE FLIES

CS-16078 EASTWARD

CHARGED PARTICLE MOTIONS IN MAGNETIC AND ELECTRIC FIELDS

X

X

ION X X

x x a x x x x x “ t i +DRIFT VELOCITY E X B/B2

X X X X

ELECTRON

cs-18299 PERPENDICULAR MAGNETIC FIELD

Figure 15

7’ N N cn w

I H H

Figure 16

..

Figure 17

VAN A L L E N RADIATION

C S-25316

r .

Figure 19

NEUTRAL POINT

VELOCITY = 500 Km/

CS-26321 Figure 20

_ .

CLASS

1- 1 2 3 3+

. -

DURATION, FREQUENCY FRACTION OF ENERGY M I N VISIBLE

HEMISPHERE Y --

5 - 20 25 4 - 43 2 PER HR 100 - 2 10 - 90 250 - 600 20 - 155 600 - 1200 lo2 MEV

2 PER YEAR > 1200 1- 40 BEV 50 - 430

THE CKEMICAL COMPOSITION O F COSMIC R A D I A T I O N H H

I M m N N

4

Z

1 2

3 , 4 , 5

6 7 8

I ELEMENT RELATIVE ABUNDANCE

I N COSMIC RADIATION

HYDROGEN HELIUM L I T H I U M , BERYL-

LIUM, AND BORON CARBON NITROGEN OXYGEN 10 < Z < 30

Z > 30

100,000 7,000

35

190 120 1 9 0 140 0.1

R E L A T I V E ABUNDANCE I N THE U N I V E R S E

100 , 000 7 , 500

3x10'3

10 15 50 30

8~10'~

Figure 21 C S -2531 4

SOLAR FLARE DATA

Figure 22

.

SOLAR FLARE INTENSITY

INTEGRAL FLUX,

PROTONS

107 ioe lo9 iolo RIGIDITY, VOLTS

Figure 23 CS-25317

RANGE IN g/CM2 VS ENERGY IN MEV F O R PROTONS

I 00 PROTON ENERGY,

MEV

I O

I .o I . I I IO 100

RANGE, g/CM2

Figure 24

C 525319

I ti H

I . C CASCADE REACTION +

EVAPORATION REACTION

€/ - N

PROTON EXCITED NUCLEUS

NUCLEUS CS -25323

Figure 25

DOSE RATE VS SHIELD THICKNESS

MATERIAL: CARBON SPECTRUM: MAY IO, 59

FLARE MEASURED 33 IO3 I k M A R Y PROTONS HRS AFTER ONSET

RAD/FLARE '02kL\ CASCADE NEUTRONS

I r r l F \\- EVAPORATION NEUTRONS

'"'E w \ SECONDARY PROTONS

0 20 40 60 80 100

G M / C M ~ Figure 26

C S -2531 8

METEOROID MASS-FREQUENCY DISTRIBUTION lOy-

CUMULATE

FREQUENCY

F'. (HITS F T 2 DAY)

SATELLITE a BALLISTIC DATA

LECTED MAXIMUM

WHIPPLE (1961)

HARVARD ITRAIL

WATSON 119561

FOR RADIATORS

I 1 I I I I I 1 10-12 IO-" IO-* IO-^ IO-' 10-2 I 102 MINIMUM METEOROID MASS, , GRAMS

Figure 27

\

FLUX PATTERN OBSERVED FROM THE EARTH

\ / I/

H Y

.BLAZER 19621

cs-26034

POINT ON THE EARTH'S ORBIT

cs-23028

' H H

1

t NUMBEfi

80 6o cs -2m -40 -2 0 0 20 40

ECLIPTIC LATITUDE Figure 29

METEOROID PENETRATION RATE EXPLORER XVI DATA - JAN 13, I963

SKIN THICKNESS, INCHES OF STAINLESS STEEL Figure a